The invention discloses a method for evaluating severity of a power system fault, which comprises the following steps of: automatically generating a fault set for a target electric network system; selecting nodes to be evaluated; carrying out electromechanical transient simulation calculation on any one fault group in the fault set, and judging if the system has instability of a power angle, a voltage and a frequency according to the simulation process information; calculating a severity coefficient of the fault according to the power angle, the voltage and the frequency respectively; weighting and combining the obtained three severity coefficients to obtain an integrated severity coefficient; repeating the above steps to finish calculation on severity coefficients of all the fault groups in the fault set; and sorting the fault groups in the fault set from big to small according to the integrated severity coefficient, and outputting a list. The method critically focuses on the influence of the power system fault on the transient stability and can quickly sort numerous fault types of the target electric network according to the severity to find out restrictive faults influencing safe and stable operation of the system.